1. Field of the Invention
The present invention relates to a force transducer for detecting compression such as a compression type load cell.
2. Description of the Related Art
A compression type load cell, which is typical of conventional force transducers, is so designed that the strain which generates in correspondence with the compression applied to a strain generator is generally transmitted to a plurality of strain gages which are pasted to the strain generator by an adhesive and electrically connected with each other in such a manner as to constitute a Wheatstone bridge circuit, and is detected as the magnitude of a voltage output which is generated in correspondence with a change in the resistance of the strain gages. As the strain gage used in such a load cell, a semiconductor strain gage made of a single crystal of a semiconductor represented by Si has recently become predominant.
FIG. 2 is a perspective view of a conventional compression type load cell which is composed of a plurality of semiconductor strain gages pasted to a strain generator by an adhesive. In FIG. 2, the reference numeral 110 represents a semiconductor strain gage pasted on the side surface 150a of a strain generator 150 by an adhesive 160 in the direction of X in which the compression W applied to the strain generator 150 acts. The reference numeral 120 represents a semiconductor strain gage pasted in the direction of Y which is the direction orthogonal to the direction of X in which the semiconductor strain gage 110 is pasted. Semiconductor gages (not shown) are also pasted on the opposite side of the side surface 150a in the same manner as the semiconductor strain gages 110 and 120. These semiconductor strain gages are connected with each other in such a manner as to constitute a Wheatstone bridge circuit in order to reduce the deleterious influence of a change in the resistance caused by a change in the temperature on the characteristics.
In the compression type load cell shown in FIG. 2, when the compression W is applied to the top surface 150b of the strain generator 150, a compression strain caused by a compression stress .sigma..sub.x which works in the direction of X through the adhesive 160 acts on the semiconductor strain gage 110, and on the semiconductor strain gage 120 the elongation strain in the direction of Y acts through the adhesive 160 in accordance with the Poisson ratio .nu.. Thus, a change of resistance is brought about due to the piezoresistive effect.
In FIG. 3, the semiconductor strain gage 110 is enlarged in order to explain the change of resistance in the semiconductor strain gage 110 in the conventional compression type load cell shown in FIG. 2. In FIG. 3, the reference numerals 111 and 111' represent a pair of opposing output electrodes provided on the crystal face 110a of the semiconductor strain gage 110 in the direction of X in which the compression W works. The reference numerals 112 and 112' represent a pair of opposing input electrodes. In FIG. 3, the output electrodes 111, 111' also serve as the input electrodes 112, 112'.
In the semiconductor strain gage 110 shown in FIG. 3, the compression stress .sigma..sub.x which is called perpendicular stress is generated by the strain of the strain generator when the compression W is applied to the strain generator 150, whereby the semiconductor strain gage 110 causes a change of resistance .DELTA.R/R represented by the formula (1) due to the piezoresistive effect. EQU .DELTA.R/R=.pi..sub.11' .multidot..sigma..sub.x ( 1)
In the formula (1), .pi..sub.11' represents a piezoresistive coefficient of the semiconductor strain gage 110 which is provided with the output electrodes 111, 111' and the input electrodes 112, 112' and to which the compression W is applied in the direction of X such that the compression stress .sigma..sub.x is generated in the direction of X, as shown in FIG. 3. A semiconductor strain gage is generally so designed as to have the piezoresistive coefficient .pi..sub.11' at its maximum under the above-described condition. For example, a semiconductor strain gage consisting of a p-type Si single crystal is so designed that the direction of X is the direction of &lt;111&gt;. A semiconductor strain gage consisting of an n-type Si single crystal is so designed that the direction of X is the direction of &lt;100&gt;.
As described above, a conventional force transducer is composed of a plurality of semiconductor strain gages shown in FIG. 3 pasted to a strain generator by an adhesive and connected with each others in such a manner as to constitute a Wheatstone bridge circuit. A compression type load cell which is typical of such a force transducer has the following problems, but no compression type load cell incorporating a novel system for detecting compression has been known for several years.